Dry eye (DE) is multifactorial disease that is often accompanied by symptoms of irritation, dryness and visual disturbances. Relief from symptoms is the primary reason patients seek medical attention; however, the basis for irritation and visual dysfunction in DE is poorly understood. Although all forms of DE present with hyperosmolar tears and most also display signs of ocular inflammation, reduction of proinflammatory molecules alone by topical treatments on the eye often are not sufficient to manage symptoms of irritation in moderate to severe DE. This project uses in vivo and in vitro approaches to test the hypothesis that central neuroplasticity is a critical, yet unexplored, factor in DE that acts t maintain symptoms of irritation and hampers treatment. In Aim 1, in a rat model for tear deficiency (exorbital gland removal), we will determine the effects of tear reduction on ocular-responsive neurons at different rostrocaudal levels of the trigeminal brainstem nuclear complex (TBNC), the initial site for synaptic integration of somatosensory signals from the eye. We will establish a quantitative sensory testing protocol appropriate for the eye and periorbital skin to assess the encoding properties of TBNC neurons and to better define possible neurological dysfunctions in DE. The TBNC is an integral component of the lacrimal functional unit and is necessary for ocular homeostasis and symptom expression in DE. Eye muscle electromyographic activity and reflex lacrimation are measured and serve as functional outputs with which to assess changes in TNBC neural activity. Aim 2 will determine if the dense longitudinal projection pathway that connects caudal with rostral portions of the TBNC is necessary for central sensitization of TBNC neurons and enhanced eye muscle activity in our DE model. Aim 3 will examine cell/molecular pathways closely associated with inflammatory pain (e.g., glutamate receptor activation, mitogen-activated protein kinase cascades and microglia activation) and determine their role in mediating persistent sensitization of TBNC neurons, enhanced eye blink behavior, eye muscle activity and reflex lacrimation in our model for DE. Successful completion of this project will establish a stimulus protocol appropriate for periocular sensory testing, portions of which may be adaptable for use in the clinic. This project will provide new information on the neurological basis for irritation and discomfort after tear reduction and will identify possible central neural mechanisms and therapeutic targets for symptom management in DE.